Various proposals have been made so far on polymerization catalysts for conjugated dienes, and the polymerization catalysts play a very important role in industrial fields. In particular, various polymerization catalysts which provide a high cis-1,4-linkage content have been studied and developed to obtain conjugated diene polymers with enhanced performance in thermal and mechanical characteristics. For example, complex catalyst systems containing as a main component a compound of a transition metal such as nickel, cobalt, and titanium are known, and some of them have already been widely used in industrial applications as polymerization catalysts for butadiene, isoprene, etc. (see Non-Patent Document 1 and Patent Document 1).
On the other hand, in order to attain a higher cis-1,4-linkage content and superior polymerization activity, complex catalyst systems which consist of a rare earth metal compound and an organometallic compound containing a group I to group III element have been studied and developed, and highly stereospecific polymerization has been studied actively (For example, see Non-Patent Documents 2–5, Patent Document 2 or the like). Among those catalyst systems, complex catalysts containing as main components a neodymium compound and an organoaluminum compound were revealed to provide a high cis-1,4-linkage content and have superior polymerization activity. The catalysts have already been used in industrial applications as polymerization catalysts for butadiene, etc. (see Non-Patent Documents 6 and 7).
With the recent progress of industrial technologies, requirements for polymeric materials as commercial products have become increasingly higher, and development of polymeric materials having still higher thermal characteristics (thermal stability, etc.) and mechanical characteristics (tensile modulus, bending modulus, etc.) has been desired strongly. As one of promising means for achieving the object, attempts have been made to produce a polymer having a high cis-1,4-configuration content in microstructure, a high molecular weight and a narrow molecular weight distribution by using a catalyst having a high polymerization activity to conjugated dienes.
For example, use of a samarocene complex as a polymerization catalyst for 1,3-butadiene and of MMAO or AlR3/[Ph3C][B(C6F5)4] as a co-catalyst in combination with the catalyst is known to provide polybutadiene, which has a highly regulated 1,4-cis-configuration and a narrow molecular weight distribution, in high yields (see Non-Patent Document 8). Moreover, living polymerization has been confirmed in systems using (C5Me5)2Sm[(μ-Me)AlMe2 (μ-Me)]2Sm(C5Me5)2/Al(i-Bu)3/[Ph3C][B(C6F5)4] as a catalyst, and a molecular weight of the produced polymer has become controllable (see Non-Patent Document 9). In addition, the compositions disclosed in Patent Document 3 are known as catalyst compositions for polymerization.
However, the development of a production method for a conjugated diene polymer or copolymer of a conjugated diene and an aromatic vinyl compound having a higher cis 1,4-configuration content in microstructure, a high molecular weight, and a narrow molecular weight distribution and the development of a polymerization catalyst used in the production method have been desired. No isoprene polymer having a sufficiently high cis-1,4-configuration content in microstructure and a narrow molecular weight distribution has been obtained, and the development of a production method for such a polymer and the development of a polymerization catalyst used in the production method have been desired.                <Non-Patent Document 1> Ind. Eng. Chem., 48, 784, 1956        <Non-Patent Document 2> Makromol. Chem. Suppl, 4, 61, 1981        <Non-Patent Document 3> J. Polym. Sci., Polym. Chem. Ed., 18, 3345, 1980        <Non-Patent Document 4> Sci. Sinica., 2/3, 734, 1980        <Non-Patent Document 5> Rubber Chem. Technol., 58, 117, 1985        <Non-Patent Document 6> Macromolecules, 15, 230, 1982        <Non-Patent Document 7> Makromol. Chem., 94, 119, 1981        <Non-Patent Document 8> Kaita, S., et al., Macromolecules, 32, 9078, 1999        <Non-Patent Document 9> Kaita, S., et al., Polym. Prepr. Jpn., 49, 211, 2000        <Patent Document 1> JP 37-008198 B        <Patent Document 2> DE 2848964 A        <Patent Document 3> PCT/JP00/1188 Specification        